The present invention relates to a deceleration-sensitive braking pressure control unit for a vehicular hydraulic brake system, using a spring-loaded stepped piston. The smaller effective surface of the piston is close to an inlet chamber connected to a braking pressure generator. The larger effective surface of the piston is close to an outet chamber that is connected to at least one wheel brake cylinder. The inlet chamber and outlet chamber communicate through a channel containing a valve constituted by a deceleration-sensitively movable closure member, such as a ball, in the inlet chamber and by a valve seat disposed at the end of the stepped piston that is closest to the valve.
A known arrangement including these features is shown in U.S. Pat. No. 3,770,326 issued Nov. 6, 1973 to H. J. Vogt. In the unit shown by that patent, the stops for the closure member and the sleeve are fixed relative to the housing. With the inlet pressure increasing gradually, the stepped piston will move towards the inlet chamber in opposition to the force of its spring due to the two separate stepped effective surfaces. The minimum distance of valve movement will have been covered at a predetermined change-over pressure; the valve will then be able to close in dependence upon the deceleration-sensitive movement of the closure member (ball). When the valve is closed, the associated second branch of the characteristic curve of the arrangement has less inclination than the first branch which is followed when the valve is opened. A support affixed to the housing is provided on the side of the closure member remote from the stop. The support's distance from the stop amounts to about double the axial length of the closure member.
In this construction, the change-over pressure, and the inclination of the branches of the pressure characteristic curve, are interdependent. This interdependence impairs the optimal dimensioning of the braking pressure control unit. If the closure member remains on the support on a malfunction, the valve will not act at all so that the entire unit fails and the danger of wheel-lock will be increased considerably during operation of the rear-wheel brakes.